A mobile computing device, such as a smart phone, contains a multi-core chip to provide computing power. Examples of processing cores include a Digital Signal Processor (DSP) core, a Graphics Processing Unit (GPU), a Central Processing Unit (CPU), a modem, and a camera core. Each core may include multiple clocks to capture, store, and transmit digital data at the rising and or falling edges of those clocks.
A clock in a digital processing core may be provided in a number of different ways. One example is to use a crystal that emits a known frequency when exposed to a voltage. Another example is a circuit that is based on a ring oscillator, such as a digitally controlled oscillator. A digitally controlled oscillator may include a power supply that uses a stable reference voltage to provide an output power to the oscillator.
Process, voltage, and temperature (PVT) variation may affect the operation of a digitally controlled oscillator. For instance, slight variance in dimensions of a transistor or doping in a transistor may cause that transistor to be either fast or slow compared to its ideal operation. Similarly, some transistors may behave fast or slow as a result of temperature changes. Also, an operating voltage of the device may affect whether transistors behave fast or slow. A given oscillator may include a multitude of transistors that are each potentially affected by some amount of variation. Accordingly, PVT variation may cause undesired effects in a digital oscillator unless effective compensation is applied.
Additionally, some conventional systems may use a current mirror circuit to provide the reference voltage to the oscillator's power supply. While the current mirror circuit may typically be expected to provide a steady reference voltage or current, some current mirror architectures may be better than others. For example, a beta multiplier may be sensitive to supply voltage variations due to channel length differences in their constituent transistors. An example conventional complementary metal oxide semiconductor (CMOS) bandgap reference employs an amplifier to create a more “ideal” current mirror that is insensitive to supply variation. However, the addition of the amplifier may result in higher power use and larger die area. Furthermore, conventional current mirrors do not generally compensate for PVT variation of transistors in downstream components, such as oscillators.
There is currently a need for a design that is capable of providing a reference voltage or current that is precise and may compensate for variation in the transistors of downstream components.